The present disclosure relates to a display, and particularly to a display in which thin film transistors are provided as switching elements for pixel electrodes and which performs active matrix display.
In a liquid crystal display and a flat panel type display using organic electroluminescence devices as light emitting devices, thin film transistors (TFTs) are used as switching elements for performing active matrix display with a plurality of pixels. The thin film transistors include TFTs using polycrystalline silicon (poly-Si) for active regions (poly-Si TFT) and TFTs using amorphous silicon (amorphous Si) for active regions (amorphous-Si TFT).
The poly-Si TFT is about 10 to 100 times higher than the amorphous Si TFT in carrier mobility and is smaller in deterioration of ON current. Thus, the poly-Si TFT has very excellent characteristics for use as a constituent material of switching elements.
As a technology of manufacturing the poly-Si TFT, the so-called low-temperature poly-Si process in which an amorphous Si film is made polycrystalline by using a low temperature process at a temperature of generally not more than 600° C. has been developed, whereby a reduction in the cost of substrates has been realized. For example, in a low-temperature poly-Si process using an excimer laser, pulsed irradiation of an amorphous Si film with a laser beam shaped into a line form is conducted while moving the irradiation spot little by little so that the successive irradiation spots overlap mostly with each other, whereby the same portion of the amorphous Si film is irradiated with the laser beam from 10 to 20 times. This makes it possible to obtain a polycrystal in with a grain diameter made uniform over the whole area of the active region.
As another example of the low-temperature poly-Si process, there has been proposed a method in which an amorphous Si film is irradiated with a continuous laser beam obtained from the higher harmonics of YAG laser while moving the irradiation spot at a constant speed so that the irradiation energy is constant, thereby forming a crystallized region, and patterning is conducted so that the region free of grain boundary becomes an active region of a thin film transistor (refer to Japanese Patent Laid-open No. 2003-77834 (particularly, paragraphs 0091 to 0092, AND 0169)).
In addition, as a method for restricting the width of lateral growth of a crystal by multi-stage irradiation using a mask, the sequential lateral solidification (SLS) has been proposed from the University of Columbia and the like (refer to A. T. Vouysas, A. Limonov and J. S. Im, “Journal of Applied Physics” (2003), Vol. 94, pp. 7445 to 7452) referred to as Non-Patent Document 1 hereafter.
In recent years, in the above-mentioned flat panel type displays, development of a display with a high frame rate has been under way for the purpose of further enhancing the motion picture characteristics and contrast characteristic. In addition, development of a novel display using auto-light-emitting devices such as organic EL devices has also been under way. Along with these developments, there has been a request for development of a thin film transistor free of degradation of characteristics even upon abrupt flow of a large current and showing small dispersion of switching element characteristics, for use as a pixel electrode switching element capable of coping with the requirements in these displays.
The above-mentioned poly-Si TFT obtained by the low-temperature poly-Si process according to the related art has the remarkable advantages of characteristics suitable for passage of comparatively large currents, a high carrier mobility, and little deterioration of characteristics. On the other hand, however, the poly-Si TFT has the problem of larger dispersions of characteristics, particularly initial threshold voltage and ON current, among the individual devices, as compared with the amorphous-Si TFT.
In order to prevent such dispersions, it has been tried, in relation to the crystallization by use of an excimer laser, to minimize the dispersions among the individual devices by using a film having similar crystals of about 300 nm, comparable to the wavelength of the laser. However, even the use of a film made polycrystalline in this manner has not been sufficiently effective for suppressing the dispersions of characteristics among the individual devices.
The reason for the above problem lies in that, in the case of crystallization by the crystallizing method using an excimer laser annealing apparatus according to the related art, it is difficult to accurately control the size of crystal grains in the poly-Si film, and uneven grain diameters would result. The unevenness of grain diameter leads to dispersion of the number of grain boundaries in the channel part of each thin film transistor (TFT), resulting in dispersions of characteristics of the TFT (refer, for example, to K. Yamaguchi et al., J. Appl. Phys., Vol. 89, No. 1, p. 590; M. Kimura et al., Jap. J. Appl. Phys. Vol. 40, Part I (2001), No. 1, and the like). Besides, this problem is serious particularly in a display using organic EL devices, since it leads to color irregularities or the like on a display part.
In addition, the dispersions of characteristics of the TFT as above-mentioned are difficult to sufficiently suppress, even by the low-temperature poly-Si process described in Japanese Patent Laid-open No. 2003-77834. This is considered to be due to the phenomenon in which the crystalline region constituting the inside of the channel is enlarged, and, therefore, the effects of the presence or absence of faults, dislocations or the like in the inside of the crystals are heavily reflected on the dispersions of characteristics. Besides, the dispersion of mobility in the poly-Si TFT obtained by application of the SLS process is 10% or more, even in the case of the optimum process, as seen from FIG. 8 of Non-Patent Document 2. This is considered to arise from the presence of numerous non-controlled grain boundaries in the crystalline region of the lateral growth portion.
Thus, there is a desire for a display having good display characteristics free of irregularities in color or luminance.